Since diamond is a substance having the highest hardness of the substances existing on the earth, a sintered material or a polycrystal containing diamond is used as a material for an abrasion resistant tool, a cutting tool or the like.
Japanese Patent Laying-Open No. 2003-292397 (PTD 1) discloses a diamond polycrystal composed of diamond that is obtained by conversion and sintering from a carbon substance having a graphite-type layered structure under ultra-high pressure and high temperature without addition of a sintering aid or a catalyst, and that has a mean grain size of diamond of less than or equal to 100 nm, and a purity of greater than or equal to 99%. Also disclosed is a method for producing a diamond polycrystal by direct conversion without addition of a sintering aid or a catalyst by putting a non-diamond carbon substance into a pressure cell equipped with a means for indirect heating, and conducting heating and pressurization.
International Publication No. 2009/099130 (PTD 2) discloses a diamond polycrystal produced by converting and sintering non-diamond carbon under ultrahigh pressure and high temperature without addition of a sintering aid or a catalyst, wherein the sintered diamond grains forming the diamond polycrystal have a mean grain size of larger than 50 nm and less than 2500 nm, and the diamond polycrystal has a purity of greater than or equal to 99% and a D90 grain size of the diamond of less than or equal to (mean grain size+mean grain size×0.9).
Japanese Patent Laying-Open No. 9-142933 (PTD 3) discloses a diamond polycrystal containing 0.1 to 30% by volume of a substance composed of an oxide and/or a carbonate and/or a carbide of a rare-earth element, and the rest of diamond.
Japanese Patent Laying-Open No. 2005-239472 (PTD 4) discloses a diamond sintered material having high strength and high abrasion resistance, containing sintered diamond grains having a mean grain size of less than or equal to 2 μm, and the rest of a binder phase, wherein the content of the sintered diamond grains in the diamond sintered material is greater than or equal to 80% by volume and less than or equal to 98% by volume; the binder phase contains at least one element selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, and molybdenum in a content in the binder phase of greater than or equal to 0.5% by mass and less than 50% by mass, and cobalt in a content in the binder phase of greater than or equal to 50% by mass and less than 99.5% by mass; part or all of the at least one element selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, and molybdenum exists as carbide grains having a mean grain size of less than or equal to 0.8 μm; the structure of the carbide grains is discontinuous; and the neighboring sintered diamond grains are bonded to each other.